Hydrophobic Hydration at Fluorinated and Hydrocarbon Self-Assembled Monolayers Investigated by In Situ Soft X-Ray Emission Spectroscopy and Molecular Dynamics Simulation

Abstract We investigated the hydrogen-bonding state and spatial distribution of water at hydrophobic self-assembled monolayer (SAM) interfaces formed by fluoroalkyl and hydrocarbon chains using in situ soft X-ray emission spectroscopy (XES) and molecular dynamics (MD) simulations under controlled humidity conditions. Under low water coverage, the hydrocarbon-based SAM exhibited spectral features attributed to isolated water molecules, while the fluorinated SAM showed signatures indicative of tetrahedrally coordinated hydrogen-bond networks. MD simulations revealed that water molecules at the fluorinated SAM interface preferentially populate the second hydration shell around polar functional groups rather than directly occupying the first shell, consistent with the XES observations. Natural bond orbital (NBO) analysis demonstrated that the inductive effect of fluorine suppresses charge localization at the amide group in the fluorinated SAM relative to the hydrocarbon-based SAM, weakening direct water–surface hydrogen bonding and promoting water–water network formation. These results suggest that fluorinated SAM interfaces enhance hydrophobicity, in part, by promoting extended tetrahedral water–water networks characteristic of hydrophobic hydration, rather than by direct water exclusion. These findings provide molecular-level insights into structure–property relationships in SAM-based materials and inform the design of water-repellent surfaces and fluorine-reduced hydrophobic coatings.